Let the density of the object be ρ.
Let the  density of the liquid be σ.
Let the volume of the object be V.
Let g be the acceleration due to gravity.
Upthrust force of the liquid is Vσg.
weright of the body is Vρg.
Given condition is Vρg=Vσg
V and g gets cancelled on both sides.
:. density of the object =density of the liquid.
:. relative density of the object =relative density of the liquid

{ as relative density = density of substance/density of water at 4degrees centigrade}
Triple point of a substance is temperature and pressure at which three phases of that substance co exist in equilibrium